The welding process using a laser beam generally is a high speed, low heat input and high efficiency welding process as is well known in the art. In this welding process, the permissible gap of the groove is very small because a very high positioning precision is required.
In the laser beam welding, a laser beam is converged through a lens or like optical system, and its intensified energy density spot is used as a heat source for the welding. A weld bead of high bead shape index (i.e., depth of penetration divided by width of bead) thus can be obtained with a low heat input. For this reason, the laser welding is extensively utilized in applications where thermal deformation due to the welding heat input is undesired. However, since a spot of a laser beam converged by a lens or like optical system is used as the source of heat, the permissible gap of the groove is very small, and the edge preparation and butt of metals to be joined require high precision.
Usually, the permissible groove gap is 15 20% of the thickness of metals to be joined, and when joining welding coils in a steel process line or like case where the weld line is long, it is difficult to maintain the groove gap within the permissible value. By way of example, when joining cold rolled steel plates with a thickness of 1 mm by welding, the permissible groove gap is approximately 0.1 mm. When a cold rolled steel plate is cut to a length of 1 m or more using an ordinary shearing machine, the cut edge is liable to have at least locally curved portions. If it is intended to join such steel plates by square groove butt welding, the groove gap is liable to exceed 0.2 mm. In such a case, local failure of deposition of the weld bead is liable when the metal pieces are joined by the laser welding, thus spoiling the soundness of the joint.
Various measures have been proposed for the purpose of increasing the permissible value of the groove gap. These proposed measures, however all have problems in the operability. In addition, the groove gap is rather increased to deteriorate the features of the laser welding, while the permissible groove gap is not improved so much.
As an example, U.S. Pat. No. 4,167,662 discloses a welding process, in which an electric arc is produced between an electrode and a heat-affected zone (hereinafter referred to as HAZ) including the weld zone produced by a laser beam directed to the metals to be joined. More specifically, as shown in FIG. 1, a laser beam 1 generated from a laser (not shown) is converged by a lens 2 so that it is focused at a point 4 on metals 3 to be joined, thus forming a HAZ at the point 4 of focus. An electric arc is produced between an electrode 5 and the HAZ. Reaction gas, e.g., oxygen, is jet from a nozzle 7 toward the point 4. The metals 3 are joined by this welding process. In this welding process, as the electrode 5 may be used those employed for the TIG welding or MIG welding. In addition, the heat source that is constituted by the laser beam may be augmented by the heat of the electric arc and further by the reaction heat of the oxygen jet, if desired. The permissible groove gap thus can be increased. On the demerit side, however, the welding heat input is inevitably increased, which is undesired from the standpoint of the low heat input welding, a primary feature of the laser welding.
FIG. 2 shows a set-up of a different proposal. In this case, a filler wire is used for multi-layer build-up welding using a high power laser of an output power of 4-KW or above to join metals 3 arranged to define a single Vee groove 8.
The groove in this set-up is not a see-through groove such as a square groove but a single Vee groove, and the filler wire is supplied for the purpose of adjusting the composition of the deposited metal only. That is, the filler wire is not supplied as a means of increasing the permissible value of the gap of a see-through groove such as a square groove.
In order to overcome the deficiencies discussed above, the inventors have conducted extensive research and investigations.
It is confirmed that the status of formation of the globule or droplet varies with the speed of feeding of the filler wire. For example, where a 2-KW CO.sub.2 laser beam is focused through a lens with a focal distance of 75 mm onto a filler wire with a diameter of 0.9 mm, only a giant globule is formed from the filler wire when the wire is fed at a speed of 0.6 m/min, whereas with a wire feed speed of 1.4 m/min or above there occurs spattering of a droplet from the wire tip as soon as it is formed.
The invention is predicated in the findings mentioned, and it seeks to provide a welding process using a laser beam for joining metals such as steel pieces along a see-through groove such as a square groove formed between opposed edges of the metals without use of any backing strip but by supplying a filler wire, in which the filler wire is inserted into the groove to a point therein coincident with the focused spot of the laser beam so that the globule or droplet produced from the filler wire properly proceeds to the opposed edge faces of metals defining the groove, thereby improving the permissible gap of the groove.